Photochromism is the reversible function of a certain compound that it changes its color upon exposure to light including ultraviolet light such as sunlight or light from a mercury lamp (color development) and returns to its original color when it is put in the dark by stopping its exposure to light (fading). A compound having this property is called “photochromic compound”. One of the uses of the photochromic compound is a light control material for sunglasses, and chromene compounds, spirooxazine compounds and spiropyran compounds have been used as the photochromic compound.
Since the chromene compounds in particular have excellent light resistance and obtain various developed colors by optimizing a substituent, a large number of studies have been made. For example, there is known a chromene compound having an indeno(2,1-f)naphtho(1,2-b)pyran structure represented by the following formula (1) (refer to International Laid-Open WO1996/14596).
In the above formula, Me is a methyl group. The same shall apply to the formulas below.Also, there is known a compound represented by the following formula (II) as a chromene compound having a short fading half period (International Laid-open WO2001/60811).
Since this chromene compound has a spiro ring bonded to the 13-position carbon atom, it has a short fading half period due to this structure.
However, even the above chromene compounds have room for the improvement of environmental dependence and temperature dependence. A description is first given of environmental dependence.
(Environmental Dependence)
The color of the chromene compound is changed by its structural change. Therefore, the chromene compound has quick optical response in an environment in which the structural change readily occurs as in a solution. However, in an environment in which the structural change hardly occurs as in a polymer solid matrix, the optical response is slow and the fading half period tends to be prolonged, that is, the optical response tends to deteriorate. This reason is considered to be that the structural change of the chromene compound is restricted due to a very small free space in the polymer solid matrix as compared with that in the solution. This problem tends to become marked particularly when the chromene compound is kneaded into a synthetic resin (polymer) having high hardness or high heat resistance.
To improve this, a method for improving the polymer solid matrix is proposed. For example, there is known a method in which a chromene compound is dispersed into a polymer solid matrix by using a curable composition comprising a monomer from which a resin having high hardness is obtained, a monomer from which a resin having low hardness is obtained and a photochromic compound (International Laid-open WO2001/005854). Also, there is known a method in which a chromene compound is dispersed into a polymer solid matrix by bonding a polysiloxane oligomer to a photochromic compound (International Laid-open WO2004/041961). Further, there is known a method in which a chromene compound is dispersed into a polymer solid matrix by using a curable composition comprising a monomer from which a resin having high hardness is obtained, a monomer from which a resin having low hardness is obtained and a photochromic compound (International Laid-open WO2001/005854).
According to these methods, even when a conventional chromene compound is used, the fading half period can be shortened to a certain extent as compared with other polymer solid matrices. However, since these methods do not aim to shorten the fading half period of the chromene compound itself, there is limitation to the improvement of the fading half period. Therefore, to further improve the optical response of a chromene compound in a polymer solid matrix, that is, to shorten the fading half period, the chromene compound itself must be improved.
(Temperature Dependence)
The chromene compound is a T type photochromic compound which returns to an achromatic state by heat. It is known that the fading half period of this photochromic compound greatly depends on temperature and becomes long at a low temperature.
For example, when the chromene compound represented by the above formula (II) is dispersed into a certain polymer solid matrix, the fading half period of the chromene compound is 50 seconds at 23° C. and 300 seconds at 10° C. Although it is known that conventional chromene compounds have a very long fading half period at a low temperature, requirements for the chromene compounds are becoming higher and the development of a compound having a short fading half period even at a low temperature has been desired.
To improve the above-described environmental dependence and temperature dependence, a chromene compound having optical response which is several times faster, that is, a fading half period which is several times shorter than that of a conventional chromene compound is needed. However, there has been unknown such a chromene compound. Since it is considered that a chromene compound which has quick optical response, that is, a short half period at a low temperature in a general-purpose polymer solid matrix, can be used for various purposes, the development of such a chromene compound has been desired.